Why resit something you should already know? Why not devote a few years to learning new things that will be very useful for you? I've never really understood resists, it seems like getting hung up on failure to me. Better to advance than get caught up, it just takes harder work at uni.
When your 16-18 its very easy to see life as a sort of career race, how quickly can I get to the highest point. By the time your 20 odd attitudes tend to change and your more likely to be grateful for where you are (however long it took to get there).